The simplest fuel cell comprises two electrodes separated by an electrolyte. The electrodes are electrically connected through an external circuit, with a resistive load lying in between them. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly, or xe2x80x9cMEA,xe2x80x9d comprising a solid polymer electrolyte membrane, or xe2x80x9cPEM,xe2x80x9d also known as a proton exchange membrane, disposed between the two electrodes. The electrodes are formed from porous, electrically conductive sheet material, typically carbon fiber paper or cloth, that allows gas diffusion. The PEM readily permits the movement of protons between the electrodes, but is relatively impermeable to gas. It is also a poor electronic conductor, and thereby prevents internal shorting of the cell.
A fuel gas is supplied to one electrode, the anode, where it is oxidized to produce protons and free electrons. The production of free electrons creates an electrical potential, or voltage, at the anode. The protons migrate through the PEM to the other electrode, the positively charged cathode. A reducing agent is supplied to the cathode, where it reacts with the protons that have passed through the PEM and the free electrons that have flowed through the external circuit to form a reactant product. The MEA includes a catalyst, typically platinum-based, at each interface between the PEM and the respective electrodes to induce the desired electrochemical reaction.
In one common embodiment of the fuel cell, hydrogen gas is the fuel and oxygen is the oxidizing agent. The hydrogen is oxidized at the anode to form H+ ions, or protons, and electrons, in accordance with the chemical equation:
H2=2H++2exe2x88x92
The H+ ions traverse the PEM to the cathode, where they are reduced by oxygen and the free electrons from the external circuit, to form water. The foregoing reaction is expressed by the chemical equation:
xc2xdO2+2H++2exe2x88x92=H2O 
Solid Oxide Fuel cells (SOFCs) operate using a mechanism similar to PEMs. The main difference is that instead of the electrolyte material comprising a polymer material capable of exchanging protons, the electrolyte material comprises a ceramic material capable of exchanging electrons.
Electrode layers are usually porous in order to allow the fuel and oxidant to flow to the electrode-electrolyte interfaces. Typical fuel cells that use porous electrode materials are bulk structures that require significant manifolding and pressures to readily deliver the fuel to the electrode-electrolyte interface. Pressing and sintering metal powders to promote adhesion, then sandwiching two such electrodes around an electrolyte layer to form the fuel cell form these porous electrodes. Connecting two or more fuel cells in series can form fuel cell stacks. During operation, such fuel cells and stacks thereof need high temperatures and pressures to provide fuel and oxidant to the electrode-electrolyte interface and to remove by-products therefrom. A need still exists for a method to fabricate porous electrodes that reduces or eliminates such high temperatures or pressures required to assist the flow of the fuel and oxidant to the electrode-electrolyte interface.
Aspects of the invention include a method comprising: forming a first dispersion by placing a plurality of rigid, inert, solid microspheres into a first liquid, depositing a first array of microspheres from the first dispersion on a surface of a host structure, removing the first liquid from the host structure, coating the deposited first array of microspheres with a layer of conductive material to form a first electrode layer, and removing the first array of microspheres to form a porous first electrode layer.
Aspects of the invention further include A method comprising: forming a first dispersion by placing a first plurality of rigid, inert, solid microspheres selected from the group consisting of glass and polystyrene having average cross-sectional diameters between about 0.1 xcexcm and about 5.0 xcexcm into a first liquid, depositing a first array of microspheres by spraying the first dispersion onto a surface of the host structure, wherein the host structure contains gas flow channels, the channels having pores of diameter smaller than the average cross-sectional diameter of the microspheres, removing the first liquid from the host structure at a temperature between about 70xc2x0 C. and about 150xc2x0 C., coating the first array of microspheres with a first layer of conductive material with a thickness between about 0.5 xcexcm and about 2.0 xcexcm, and removing the first array of microspheres to form a porous first electrode.
Aspects of the invention further include a method comprising: forming a first dispersion by placing a plurality of rigid, inert, solid microspheres selected from the group consisting of glass and polystyrene having cross-sectional dimensions between about 0.1 xcexcm and about 5.0 xcexcm into an aqueous liquid, depositing an array of the microspheres from the dispersion by spraying the dispersion onto a surface of an electrolyte layer of a fuel cell, removing the liquid from the electrolyte layer by drying at a temperature between about 70xc2x0 C. and about 150xc2x0 C., coating the deposited array of the microspheres with a first layer of conductive material with a thickness between about 0.5 xcexcm and about 2.0 xcexcm using a thin film deposition technique, and removing the array of the microspheres to form a first porous electrode.